Bacillus
anthracis, the aetiological
agent of anthrax, is a Gram-positive, spore-forming,
extracellular bacterium. The spore is both the form of
persistance in the environment and the infectant form.
Two toxins, and a poly-glutamic acid, antiphagocytic,
capsule are the main virulence factors

Spore
surface

The exosporium is the
most external structure of the spore. It consists of a
paracrystalline basal layer and of BclA, a collagen-like
glycoprotein. Other spore components required for the
assembly of the paracrystalline layer have been
identified.

Metabolic
pathways and biodiversity

Among the main
groups defined world-wide for B. anthracis strains, those of the sub-group B2, mostly found
in France, exhibit specific sugar utilisation. A1 strains
use starch but not gluconate, and conversely for B2
strains. A comparative genomic analysis has revealed
mutations affecting enzyme activities potentially
involved in sugar utilisation.

Vegetative form
surface

Four proteins are
necessary for capsule synthesis. A fifth protein
catalyzes the capsule covalent anchoring to the
peptidoglycan.

“Sortases” catalyze the covalent anchoring
of proteins harboring an “LPXTG”
motif. B.
anthracis possesses three
sortases and 14 “LPXTG” proteins. We have
constructed mutants of each of the sortases. We have
characterized an “LPXTG” protein that is
anchored by sortase A. We are defining B. anthracis sortase repertoires.

Regulation of virulence
factors synthesis

Toxin components
are maximally synthesized at the end of the exponential
phase. Transition state regulators, including CodY,
repress gene expression during exponential phase. CodY
implication in toxin, capsule and AtxA, a
B. anthracis master regulator, gene expression is under
study. B.
anthracis stringent
response has been characterized. There is a single
rel gene in B.
anthracis.

Animal models

In
vivo real time analysis of
bioluminescent B.
anthracis dissemination
during cutaneous, inhalational and gastrointestinal
infection in mice (Figure) has shown the respective role
of capsule and toxins. This led to identification of
previously undescribed portals of entry and target
organs. Local germination occurs prior dissemination to
lymph nodes.

Innate and adaptive
immunity

We have shown that
group IIA phospholipase A2, secreted by
macrophages controls
B. anthracis infection. Toxins inhibit production of this
enzyme. The cell transduction pathways involved in this
inhibition are analysed.

A non-living anthrax
vaccine composed of protective antigen and inactivated
spores has been devised for human use. We have shown that
the protective immunity afforded by spore immunisation is
dependent on cellular immunity involving CD4 T
lymphocytes and gamma interferon.